In recent years, it has been important to further reduce the particle diameter of powder toners for electrostatic image development in order to improve image properties such as resolution. Thus, various technical developments have been made. Most of powder toners for development of electrostatic image commercially available at present have a volume-average particle diameter of from about 8 to 13 .mu.m. Powder toners having the smallest particle diameter have a particle diameter of about 7 .mu.m (as measured by means of Coulter Multisizer).
Images outputted from the current electrophotographic copying machines or printers have far poorer quality than lithographic print or silver salt system photograph. For the part of toner, it is effective to develop a toner having an even smaller particle diameter and an excellent triboelectricity in order to improve these image properties. Such a toner has thus been keenly desired.
The processes for the preparation of toners can be roughly divided into two groups, i.e., dry process and wet process. Examples of the dry process for the preparation of toners include pulverization process. Examples of the wet process for the preparation of toners include polymerization process, and so-called phase inversion emulsification process as disclosed in JP-A-5-66600 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") It is said that the minimum allowable particle diameter of toners obtained on an industrial basis by pulverization process using the current crushing machine is about 7 .mu.m. Of course, toners having a particle diameter as small as about 5 .mu.m can be produced. However, the production of such a small particle diameter toner is not practical because it adds to production cost and is liable to deterioration of triboelectricity or powder fluidity accompanying the reduction of the particle diameter of toner particles.
It is said that a wet process such as polymerization process and emulsion process essentially finds no difficulty in the reduction of the particle diameter of toner particles. However, the development or production of the prior art wet process toners is mainly intended to replace crushed toners having a particle diameter of from about 7 to 13 .mu.m. Small particle diameter toners having a volume-average particle diameter of not more than about 6 .mu.m have been so far known only in fragments. No practical formulations have been known.
U.S. Pat. No. 5,691,095 discloses inventions of spherically particulate toner and process for the preparation thereof. However, no studies were made of the range of concentration of colorant. The values of concentration of colorant described in the examples in the above cited US patent are all lower than that of the present invention. Thus, the toner according to the above cited US patent doesn't exert the effect of the, present invention.
An object of the present invention is to provide a novel toner for use in the development of an electrostatic latent image in an electrophotographic printer or copying machine which provides an excellent image quality and can be consumed in a drastically reduced amount per page of printing paper.